M. Briggs

Compton-backscattered annihilation radiation from the Galactic Center region

On 1989 May 22, the High Energy X-ray and Gamma-ray Observatory for Nuclear Emissions, a balloon-borne high-resolution germanium spectrometer with an 18-deg FOV, observed the Galactic Center (GC) from 25 to 2500 keV. The GC photon spectrum is obtained from the count spectrum by a model-independent method which accounts for the effects of passive material in the instrument and scattering in the atmosphere. Besides a positron annihilation line with a flux of (10.0 +/- 2.4) x 10 exp -4 photons/sq cm s and a full width at half-maximum (FWHM) of (2.9 + 1.0, -1.1) keV, the spectrum shows a peak centered at (163.7 +/- 3.4) keV with a flux of (1.55 +/- 0.47) x 10 exp -3 photons/sq cm s and a FWHM of (24.4 +/- 9.2) keV. The energy range 450-507 keV shows no positronium continuum associated with the annihilation line, with a 2-sigma upper limit of 0.90 on the positronium fraction. The 164 keV feature is interpreted as Compton backscatter of broadened and redshifted annihilation radiation, possibly from the source 1E 1740.7-2942.

Discovery of a transient MeV range gamma-ray source

The University of California, San Diego (UCSD)/MIT hard X-ray and gamma-ray instrument on the HEAO 1 surveyed the region near the Galactic center 3 times during its lifetime in 1977-1979. During the 1977 September-October scan, a gamma-ray source was detected south of the Galactic center. The source was below the threshold sensitivity in the spring and fall of 1978. The source was detected with the medium energy phoswich scintillation counters which operated over the 80 keV-2 MeV range, had an area of 42 sq cm each, and a 17 deg FWHM aperture. The error box for the source is centered on l = 2.4 deg, b = -12.2 deg, with a 90% confidence error circle of approximately 3.5 deg radius. The flux in the 333-635 keV range was (1.89 +/- 0.29) x 10(exp -5) photons/(sq cm s keV) and was constant within statistics during the 1 month period the source was in the field of view. The spectrum can be characterized as a Gaussian in the range 300 less than or = E less than or = 650 keV, with a FWHM of 249 +/- 51 keV centered on 461 +/- 22 keV. The flux of this broad Gaussian is (6.6 +/- 1.1) x 10(exp -3) photons/(sq cm s). The source is tentatively identified with the 5.57 hr period low-mass X-ray-emitting binary system 1H 1822-371. Assuming this is correct, the ratio of gamma-ray to X-ray luminosity during the outburst was about 5; at a distance of 8 kpc, the gamma ray luminosity is 4 x 10(exp 37) ergs. The emission may be interpreted as a positron-pair plasma ejected from a compact object, possibly a black hole, and annihilating in a thick accretion disk surrounding the object.

Observation of SN 1987A with the gamma-ray spectrometer HEXAGONE

The HEXAGONE balloon-borne spectrometer was flown from Alice Springs (Australia) on 1989 May 22. HEXAGONE is a high-resolution gamma-ray spectrometer and consists of an array of twelve cooled germanium detectors (field of view 19° at 511 keV). One of the observed targets was the supernova 1987A and it was seen during 9.9 hr, 818 days after the initial optical outburst. No significant hard X-ray or gamma-ray emission is detected in the final spectrum of SN 1987A

An observation of annihilation radiation from the Galactic Center region

The Galactic Center region was observed on 22 May 1989 with a high resolution gamma-ray spectrometer flown from Alice Springs, Australia. The instrument contained an array of 12 cooled Ge detectors, each 5.5 cm dia x 5.5 cm long, with an energy resolution of 2.2 keV at 0.5 MeV, which were collimated to 18° FWHM. The observation was made at approximately 4 gm/cm2 atmosphere depth for 6 hours in a series of target and background pointings lasting 20 minutes each. Results from multiparameter Gaussian fits to the data are: 511 keV flux = (8.9 ± 2.7) x 10−4 ph/cm2-sec, line width of 1.1 −1.1+1.4 keV, and < 3.2 keV FWHM at 95% confidence. If the flux is interpreted as diffuse galactic emission, the 24° effective aperture for a uniform source gives a flux of (2.12 ± 0.64) x10−3 ph/cm2-s-rad, consistent with the SMM observations of diffuse galactic flux. The flux and width are both less than the October 1988 measurements with a 17° FWHM instrument which were obtained when the compact source at the galactic center was known to be “on”. Thus the variable compact source in the Galactic Center region may be responsible for the wider 511 keV line emission. The line width limit is consistent with annihilation in the warm (104 K) phase of the interstellar medium.

A transient MeV range gamma‐ray source observed by HEAO–1

The UCSD/MIT hard X‐ray and gamma‐ray instrument on the HEAO–1 surveyed the region near the galactic center three times during its lifetime in 1977–1979. During the Sept.–Oct. 1977 scan, a gamma‐ray source in the 300–650 keV range was detected south of the galactic center. The source was below threshold sensitivity in the Spring and Fall of 1978. The source was detected with the medium energy phoswich scintillation counters which operated over the 80 keV–2 MeV range, had an area of 42 cm2 each, and a 17° FWHM aperture. The source is centered on lII=−2.3°, bII =−12.1°, with a 905 confidence error circle of ∼3.5° radius. The flux in the 333–583 keV range was (1.98±0.35)×10−5 ph/cm2‐sec‐keV and was constant within statistics during the one month period the source was in the field of view. The spectrum can be characterized as a Gaussian in the range 300≤E≤650 keV with a FWHM of 290±60 keV centered on 490±30 keV. The source is tentatively identified with the 5.57 hr period low mass X‐ray emitting binary system...

An observation of the Galactic center region with the HEXAGONE high resolution gamma‐ray spectrometer

The galactic center region was observed for 6 hours on 22 May 1989 from a high altitude balloon with the HEXAGONE high resolution gamma-ray spectrometer. The instrument had a 285 cm{sup 2} array of cooled germanium detectors with an energy resolution of 2.2 keV at 511 keV and an 18{degree} FWHM field of view. 511 keV gamma-rays from electron-positron annihilation and 1809 keV gamma-rays from the radioactive decay of {sup 26}Al were observed to have fluxes of 8.9{times}10{sup {minus}4} and 1.9{times}10{sup {minus}4} ph/cm{sup 2}-s, respectively. Continuum emission was detected from 20 to 800 keV and preliminary results have been obtained for the spectrum. Below 120 keV this is well described by power law with a slope of {minus}2.6. In the 120--250 keV band the spectrum contains a broad line-like feature with a flux of (2 to 6){times}10{sup {minus}3} ph/cm{sup 2}-s, depending on the assumed underlying continuum. This is interpreted as the result of Compton backscattering of {similar to}511 keV photons from a compact source of electron-positron annihilation radiation.

Observation of the 511 keV annihilation line in the direction of the galactic center with HEXAGONE

The HEXAGONE balloon‐borne spectrometer has flown on 22 May 1989. HEXAGONE is a high resolution gamma‐ray spectrometer and consists of an array of twelve cooled germanium detectors. One of the observed targets was the Galactic Center and its vicinity (field of view 19° at 511 keV) and it was seen during 6.3 hours. The 511 keV annihilation line was observed with a flux of (8.88±2.67)×10−4 γcm−2 s−1, a width 1.09+1.38, −1.09 keV and its centroid at 511.54±0.38 keV. The results are consistent with an upper limit of 8.3×104 K for the temperature of the annihilation medium of the positrons.The HEXAGONE balloon‐borne spectrometer has flown on 22 May 1989. HEXAGONE is a high resolution gamma‐ray spectrometer and consists of an array of twelve cooled germanium detectors. One of the observed targets was the Galactic Center and its vicinity (field of view 19° at 511 keV) and it was seen during 6.3 hours. The 511 keV annihilation line was observed with a flux of (8.88±2.67)×10−4 γcm−2 s−1, a width 1.09+1.38, −1.09 keV and its centroid at 511.54±0.38 keV. The results are consistent with an upper limit of 8.3×104 K for the temperature of the annihilation medium of the positrons.

Observations of diffuse galactic positron annihilation radiation

Abstract The Galactic plane was scanned nearly three times by the UCSD/MIT Hard X-Ray and Low Energy Gamma-Ray Experiment on HEAO-1 from August 1977 through September 1978. Its Medium Energy Detectors were of the NaI/CsI phoswich type and operated over the 100 keV to 2 MeV range, with a 17° FWHM field of view and a 9% energy resolution at 511 keV. Sky maps for each epoch of observation were constructed in several energy bands. After subtraction of known point sources, a component associated with the galactic plane remains, whose spectrum is consistent with a power law and a positron annihilation spectrum. In the 333 to 583 keV energy band the flux is concentrated within±35° of the galactic center, and the ratio of flux/radian (anticenter) is high, with a 2σ lower limit of 13. The parameters of the galactic center regions annihilation spectrum are positronium fraction of 0.9±0.1 and 511 keV flux of (2.0±0.7)×10 −3 photons/cm 2 -sec-rad.

An Observation of SN1987A with a New High Resolution Gamma-Ray Spectrometer

The discovery (Matz et al. [1]) of gamma-ray line emission at 847 and 1238 keV from radioactive 56Co in the recent supernova SN1987A proved that explosive nucleosynthesis occurred in this supernova. Gamma-ray light curves derived from these and subsequent observations (Cook et al. [2], Mahoney et al. [3], Sandie et al. [4], Rester et al. [5], Teegarden et al. [6]) have a broad plateau from August 1987 to October 1988, with an 847 keV flux of ~ 7×l0-4 ph/cm2-sec (Tueller et al. [7]). The early detection of gamma rays required the inclusion of mixing or clumping in the models, (e.g. Pinto and Woosley [8] and Chan and Lingenfelter [9]). The gamma-ray fluxes are predicted, e.g. Bussard et al. [10], to peak at about day 400 and then decrease, by a factor of ~ 6 at day 800, as the effect of increasing transparency becomes dominated by radioactive decay. Then they should depend primarily on the amount of 56Co produced and little on the degree on mixing since most of the 56Co should be exposed. Thus measurements of the 56Co gamma-ray line fluxes and profiles will continue to be important during the decline of SN1987A.